Reinforced nancomposites and method of producing the same

ABSTRACT

A composite material having polymeric resin with disperse phases of reinforcing fibers and nanoparticle materials and its manufacture is disclosed herein. The nanoparticles may be bound together and added to the polymeric resin as microscale aggregations, and then unbound to create a disperse phase of nanoparticles in the resin. In other embodiments, the nanoparticles may be bound to a substrate, such as long fibers, and added to a polymeric resin. The nanoparticles are then unbound from the substrate and dispersed throughout the polymeric resin. The polymeric resin may have multiple components where one component may control the dispersion of the nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. application Ser. No.11/550,575, now U.S. Pat. No. 8,143,337, filed Oct. 18, 2006, whichclaims priority to and is a non-provisional of U.S. Application No.60/727,723, filed Oct. 18, 2005, both of which are incorporated byreference in their entirety as if fully rewritten herein.

TECHNICAL FIELD

The invention relates generally to plastics reinforced with acombination of long fibers and nanoparticles. More particularly, theinvention relates to improvements in the known technology forcontrolling the dispersion of the nanoparticles in the continuouspolymer matrix.

BACKGROUND OF THE INVENTION

Among the various thermoset polymers, epoxy resins provide superioroverall performance, such as good mechanical properties, chemicalresistance, and low shrinkage. Therefore, epoxy resins arehigh-performance systems for use in coating, adhesive, civilengineering, structural, electronic and composite applications.

In 1907, phenolic resins were the first thermoset resins to besynthesized commercially. These resins have excellent fire performance,good dimensional stability, excellent thermal insulation properties, andare cost effective. These properties enable phenolics to be used inhousehold appliances, business equipment, wiring devices, automotiveelectrical systems, and mass transit.

Fiber reinforced plastics (FRPs) are the most widely used composites. Ingeneral, fibers are the principal load-carrying members, while thesurrounding plastic matrix keeps the fibers in the desired location andorientation, acting as a load transfer medium. The plastic also protectsthe fibers from environmental damage due to exposure to elevatedtemperature and humidity. Fiber-reinforced composites have low specificgravity, high internal damping, and high strength-to-weight ratio andmodulus-to-weight ratio. There have been numerous applications for FRPs,and additional applications are continually sought, due to theattractive properties. In the FRP art, the terms “long fiber” and “shortfiber” are commonly used to designate fibers and will be understood bythose of skill in the art.

A “nanocomposite” is a composite containing a disperse material with atleast one dimension that is smaller than about 100 nm in size. Due tothe nanoscale dispersion and the high aspect ratios of the inorganicclays, polymer-layered silicate nanocomposites (PLSNs) exhibit lightweight, dimensional stability, heat resistance, and a certain degree ofstiffness, barrier properties, improved toughness and strength with farless reinforcement loading than conventional composite counterparts. Thesynthesis and characterization of PLSNs has become one of the frontiersin materials science.

Since the discovery of carbon nanotubes (CNTs), many people have studiedthe properties of polymer-carbon nanotube composites. However, the highcost and low volume of production of the CNT have greatly limitedproduct development and application. Carbon nanofibers (CNF), defined ascarbon fibers with diameters of up to 200 nm (and typically in the100-200 nm range) and lengths of up to about 100 microns, may serve as asubstitute for the carbon nanotubes. Recent studies indicate thatpolymer-carbon nanofiber composites (PCNFCs) have properties similar topolymer-carbon nanotube composites. These CNF nanocomposites can be usedto make conductive paints, coatings, films, tubes, sheets, and parts forelectrostatic painting, electro-magnetic interference and electro-staticdischarge applications. In addition, these composites also provideimproved strength, stiffness, dimensional stability and thermalconductivity. It makes the PCNFCs a very promising material for a widerange of applications in automotive, aerospace, electronic and chemicalindustries.

Although fiber-reinforced plastics (FRP) have good mechanicalproperties, an interface exists between the polymer matrix and theindividual fibers. This interface, which represents a substantial area,is subject to diffusive attack by water and other small molecules. Thiscan cause a substantial drop in interfacial strength and failure ofadhesion between the components. Under tension, compression, shear, orimpact, failure of the polymer matrix may also take place.

A problem continually encountered in preparing nanocomposites has beenthe difficulty in properly dispersing the nanoparticles in thecontinuous polymer phase. For example, it is often difficult to loadmore than about 10 weight % of nanoparticles into the continuous phase.By way of comparison, a highly-loaded FRP can often contain greater than50 weight % of the disperse fiber phase. Not unexpectedly, theenhancement of mechanical properties of the continuous phase polymer ina typical PLSN or PCNFC is relatively low compared with the enhancementof mechanical properties in a typical FRP. When this is combined withthe high cost of the nano-scale material, it would be surprising ifnanocomposites would have already made significant market penetration.

It is therefore an object of the present invention to provide acomposite material that effectively combines advantages of a dispersefiber phase and a disperse nanoparticle phase.

SUMMARY OF THE INVENTION

This and other objects are achieved by the exemplary embodimentspresented in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood through reference to theexemplary embodiments disclosed herein, wherein identical parts areidentified by identical part numbers, and wherein:

FIG. 1 is an x-ray diffraction pattern showing the effects of addingpoly(vinyl acetate) to composites containing long fibers and nanoclaysin a continuous phase of unsaturated polyester resin;

FIGS. 2A, 2B are microphotographs of an unsaturatedpolyester—nanoclay—long fiber composite material, FIG. 2B being anenlargement of a portion of FIG. 2A;

FIG. 3 is a diagram showing the improved flexural modulus obtained byadding poly(vinyl acetate) to an unsaturated polyester—nanoclay—longfiber composite material;

FIG. 4 is a scanning electron microscope image of a phenolicresin—carbon nanofiber—long glass fiber composite.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In an exemplary embodiment of the invention, the advantages of both FRPand polymer nanocomposites are combined to produce a composite havingpreviously unexpected properties. The composite has a substantiallycontinuous matrix of a polymer, especially a polymer resin, such as anepoxy or a phenolic resin. Two distinct disperse phases are present in asubstantially uniform manner in the matrix. The first disperse phasecomprises reinforcing fibers, such as are commercially available and aremanufactured from glass, carbon or aramid. The second disperse phasecomprises nanoclays, carbon nanofibers or carbon nanotubes. As describedin more detail below, the polymer matrix will generally comprise apredominant amount of a first, generally hydrophobic, component and aneffective amount of a second component, the second component selected tomoderate the hydrophobicity of the first component.

Speaking generally, reinforcing fibers, especially long glass or carbonfibers embue the composite with good mechanical properties. Betweenglass and carbon fibers, the glass fibers are more hydrophilic. Thenanoparticles improve the barrier properties and strengthen the matrixbetween long fibers in order to reduce the matrix failure in thecomposites and extend the longevity of the composites. Between nanoclaysand carbon nanofibers, the nanoclays are more hydrophilic.

To succeed in producing these new composites, two questions need to beanswered. How to well disperse nanoparticles into FRPs and whether theresin processability can be maintained with the presence ofnanoparticles? Speaking generally, phenolic resins or unsaturatedpolyester (UP) resins containing poly(vinyl acetate) (PVAc) as a lowprofile additive are of higher hydrophilicity than the epoxy resins orunsaturated polyester resins lacking the poly(vinyl acetate). Therefore,a variety of combination of these different materials may be selected tocontrol the distribution and dispersion of nanoparticles in the FRP toachieve desirable composite properties and processability.

In an exemplary scheme, raw nanoclay is dispersed easily into thephenolic resin, so a phenolic-clay nanocomposite is synthesized. Then,reinforcing fibers of long glass or carbon fibers are integrated intothe phenolic-clay nanocomposite. This step, producing a hybrid compositecan be achieved using a known prepreging or resin transfer molding (RTM)process. The hybrid composite obtained has long carbon fibersreinforcing a phenolic matrix having well-dispersed nanoclay. Since theadded nanoparticles tend to increase the resin viscosity, especiallywhen good dispersion is achieved at a high loading level of thenanoclay, resin processing may become quite difficult in preparingprepregs for autoclave molding or in RTM mold filling. If the nanoclaydispersion is controlled to initially limit the amount of dispersion,the resin viscosity may remain low enough to facilitate prepregpreparation or RTM mold filling. In such a system, the nanoclays formmicro-scale aggregates that are not well dispersed by a binder. Aftermolding, many techniques are useful to exfoliate the nanoclayaggregates. Among these techniques are: elevated temperature to melt thebinder, ultrasonic energy, or a combination of both. Ultrasonic energyis also useful in improving the prepreg consolidation. This extraadvantage may further improve the product properties.

In a further exemplary scheme, the same approach can be applied toorganoclays or CNFs in epoxy resins. In such an instance, thenanoparticles, such as a CNF, are bound to the reinforcing fibersurface, such as a glass surface. After this, a pure resin, such asepoxy, is integrated with the reinforcing fibers though the knownprepreg formation or RTM mold filling process, essentially ignoring thepresence of the CNF on the reinforcing fiber surface. When curing thecomposites at elevated temperatures, the binder between the CNF and thereinforcing fibers would melt, with the CNFs diffusing from thereinforcing fiber surface into the epoxy matrix. This is because theCNFs and epoxy resins are hydrophobic, while the glass surface ishydrophilic, so the CNF prefers to diffuse into the more compatibleepoxy matrix. This second method has the advantage of using a compositemanufacturing process, including prepregs preparation and molding, thatis the same as that without any nanoparticles. However, thenanoparticles have to be much more compatible with the matrix resin thanwith the reinforcing fibers to effect good dispersion. Again,nanoparticle dispersion in the polymer matrix may be enhanced byapplying ultrasonic energy.

This second approach is also applicable to a system comprising longcarbon fibers and raw clays with more hydrophilic resins, such asphenolics. Mechanical and thermal properties of such composites will becompared with both long fiber-reinforced composites and polymernanocomposites. Material composition, binder selection, and processingconditions will be optimized to achieve the best properties of thehybrid composites. Rheological measurements, kinetic analysis,permeability measurements, and mold filling observations for resinscontaining nanoparticles will be carried out to quantify the compositeprocessability during molding and curing. XRD, SEM, and TEMcharacterizations will also be performed to determine the nanoparticledispersion in the molded composites.

Attention is now directed to FIG. 1, which presents x-ray diffractionstudies of two different systems incorporating the aspects of one of theinventive embodiments. Four distinct traces are illustrated in FIG. 1.In both cases, the polymeric matrix is an unsaturated polyester (“UP”).In the first case, shown as line 12, glass fibers coated with a nanoclayare dispersed in the UP, in the absence of poly(vinyl acetate). Theexpected diffraction peak for the clay (in the zone between 2 and 3 onthe abscissa) is not seen in line 12, indicating that the nanoclay hasstayed at the glass fiber surface and has not dispersed into the UPmatrix. When the identical system is prepared again, but the polymericmatrix includes 3.5% PVAc, line 16 is obtained, showing the diffractionpeak indicative of diffusion of the nanoclay into the polymeric matrix.In a second case, shown as line 14, carbon fibers are coated with thenanoclay and dispersed in the UP, in the absence of poly(vinyl acetate).The expected diffraction peak for the clay is not seen in line 14,indicating that the nanoclay has stayed at the carbon fiber surface andhas not dispersed into the UP matrix. When the identical system isprepared again, but the polymeric matrix includes 3.5% PVAc, line 18 isobtained, showing the diffraction peak indicative of diffusion of thenanoclay into the polymeric matrix.

Further confirmation of this phenomenon is provided in FIGS. 2A and 2B.A photomicrograph of the type of system shown in line 12 of FIG. 1 ispresented, with a glass fiber visible near the bottom of FIG. 2A. A barat the top of FIG. 2A, indicating a length of 3.2 microns, shows thescale of the image. FIG. 2B, which is enlarged from the image of FIG. 2Aat the point shown near the glass fiber, demonstrates that the nanoclayparticles have stayed near the glass fiber. In FIG. 2B, the bar shows alength of 160 nm, to illustrate scale. This shows that the absence ofPVAc in this system may improve the barrier properties of the material,by preventing small molecules, such as water, from diffusing into aninterface between the long fibers (glass, in this example) and thepolymeric matrix.

Some further aspects of the UP-glass fiber—nanoclay system are shown inFIG. 3. In a system without nanoclay being present (bars 22 and 24 inFIG. 3), addition of 3.5% PVAc to the system has little effect on theflexural modulus, and, if anything, the flexural modulus is reducedslightly. When clay is added (bars 26 and 28 in FIG. 3), the presence ofPVAc markedly improves the flexural modulus, as seen by comparing bars26 and 28. Comparing systems with or without nanoclay, that is,comparing bar 22 to bar 26 or comparing bar 24 to bar 28, it is readilynoted that the presence of nanoclay makes little difference when PVAc isabsent, but the addition of nanoclay greatly improves flexural moduluswhen the PVAc is present in the UP polymer matrix.

In another example, a system involving phenolic resin with dispersecarbon nanofibers and long glass fibers is demonstrated by way of ascanning electron micrograph presented as FIG. 4. In this system, wherea pair of the long glass fibers are shown by arrows, the carbonnanofibers are seen to be well dispersed into the phenolic matrix thatacts as the continuous phase.

Other embodiments and uses of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. The specification and examples shouldbe considered exemplary only and do not limit the intended scope of theinvention.

What is claimed is:
 1. A method for preparing a composite materialincluding a polymer resin in which fibers and nanoparticles aredispersed, comprising the steps of: mixing said nanoparticles, in anaggregate form, and said polymer resin to form a nanocomposite; allowingsaid nanoparticles to disperse from the aggregate form into micro-scaleaggregates within said nanocomposite such that said nanocomposite has aviscosity suitable for molding; molding said nanocomposite beforeexfoliating said micro-scale aggregates in said nanocomposite; addingsaid fibers to the nanocomposite during molding to form a hybridcomposite; and exfoliating said micro-scale aggregates within saidhybrid composite after molding.
 2. The method of claim 1 wherein theexfoliating step is achieved by applying ultrasonic energy.
 3. Themethod of claim 1 wherein at least one of said nanoparticles is bound toat least one of said fibers.
 4. The method of claim 1 wherein saidfibers are selected from the group consisting of glass, carbon, aramid,and combinations thereof.
 5. The method of claim 1, wherein saidnanoparticles are selected from the group consisting of nanoclays,carbon nanofibers, carbon nanotubes, and combinations thereof.